This invention relates to curable organopolysiloxanes having epoxy groups. This invention also relates to a method for synthesizing these curable organopolysiloxanes having epoxy groups and to the use thereof as curable coating materials with abhesive properties, as casting compositions and as coating materials for glass fibers.
There is extensive patent literature which deals with curable epoxysiloxanes and their use as coating agents. Of the large number of patents, U.S. Pat. No. 4,313,988 is named initially. In this patent, a product is described which consists of a support tape with a pressure-sensitive adhesive. On one side of the tape, a layer of a normally adhering and pressure-sensitive adhesive is provided and on the other surface, a material with abhesive properties is disposed. This material consists of a curable epoxypolysiloxane either in solution or in 100% form, which corresponds to the formula ##STR1##
In this formula
R represents a low molecular weight alkyl group with 1 to 3 carbon atoms; PA0 R.sup.1 represents a monovalent hydrocarbon group with 4 to 20 carbon atoms; PA0 E is a monovalent hydrocarbon group containing epoxy groups; PA0 M is a silyl group having the formula R.sub.3 Si--; R.sub.2 R.sup.1 Si--or R.sub.2 ESi--, R, R.sup.1 and E being as defined above; PA0 a has a value of 5 to 200; PA0 b is 0 or has a value up to 20% that of a and a +b is 5 to 200; PA0 c can assume the value 0 when M is the R.sub.2 ESi group, or is larger than 0 but smaller than 20% of the value of a+b when M is the R.sub.3 Si--, R.sub.2 R.sup.1 Si--or R.sub.2 ESi--group and PA0 n has a value of 1 to 75, PA0 R.sub.2 is selected from PA0 a has a value of 1 to 1,000 and PA0 b has a value of 0 to 10, PA0 R is an alkyl group with 1 to 8 carbon atoms, PA0 R' is R or a monovalent, epoxy-functional organic group with 2 to 20 carbon atoms, with the proviso that at least two R' groups are monovalent, epoxy-functional organic groups and PA0 x has a value of 0 to 20. PA0 R.sub.4 is selected from: PA0 a has a value of 1 to 1,000 and PA0 b has a value of 0 to 1, PA0 R.sub.4 is selected from: PA0 a has a value of 1 to 1,000 and PA0 b has a value of 0 to 1,
with the proviso that the monovalent hydrocarbon group E, which contains epoxy groups, contains at least one polymerizable epoxy group of the formula ##STR2##
The rest of the E group consists of carbon and hydrogen atoms and is free of acetylenic unsaturations. In addition to the oxiran ether oxygen, a carbonyl oxygen or the ##STR3## group can be present.
Also, the preparation can contain up to 98% by weight, based on epoxypolysiloxane, of an epoxysilane.
It is pointed out in the aforementioned U.S. Pat. No. 4,313,988 that, according to the state of the art, coatings with abhesive properties, that is, with release properties towards adhesives, are widely used. Dimethylpolysiloxanes provide coatings from which an adhesive tape can be pulled off with the low release force of 4 to 16 g/cm of tape width This measure indicates the force which must be employed to pull a 1 cm wide adhesive tape from the abhesively finished surface. If, however, adhesive tapes are to be produced, for example, in roll form and to be finished abhesively, this degree of abhesiveness is too high, since it leads to instabilities of the coiled roll. For such purposes, coatings are desired, the release forces of which, with respect to adhesive tapes, are selective and can be increased, for example to 60 to 350 g/cm and particularly to about 60 to 200 g/cm.
The epoxysiloxanes contain at least one polymerizable epoxy group of the formula ##STR4##
The following can be named as examples of such epoxy groups: ##STR5##
The epoxy groups preferably are located terminally at the hydrocarbon group.
Preferred epoxypolysiloxanes of U.S. Pat. No. 4,313,988 are those of the formula ##STR6## wherein R is a methyl group, b has a value of 0 and the E group is a .beta.-(3,4-epoxycyclohexyl)ethyl or a .gamma.-glycidoxypropyl group.
Curing of epoxypolysiloxanes or preparations which contain such epoxypolysiloxanes is accomplished by addition and in the presence of conventional epoxy curing catalysts. Examples of such catalysts are tertiary amines, Lewis acids, such as BF.sub.3, and their complexes, such as those with ethers or amines, and polyaromatic iodonium and sulfonium complex salts, which contain SbF.sub.6 --or BF.sub.4 --anions as anions. Organic acids and their salts or derivatives can also be used as curing agents, such as perfluorinated sulfonic acids.
Suitable curing agents are also described in U.S. Pat. Nos. 4,394,403 and 4,310,469.
Very frequently, however, when adding the curing agent, it is observed that, depending on the structure of the preparation and particularly of the siloxanes, the miscibility and solubility of the photoinitiators (curing agents) are limited. Epoxy-functional siloxanes have already been modified in order to improve their compatibility with onium salt photoinitiators. For Example, in EP-A-0 334 068, epoxy-functional siloxanes are described, a portion of the epoxy groups of which was esterified by reaction with aromatic acids, such as benzoic acid, or unsaturated acids, such as acrylic acid. By these means, the compatibility of the epoxysiloxanes with the curing catalysts was increased. At the same time, however, the content of curable epoxy groups was decreased. Epoxy groups are reactive groups which can be activated to polymerize by weaker acids. This makes it exceedingly difficult to control the reaction. In the extreme case, the reaction can become uncontrolled and lead to gelling. Moreover, residual acid contents must be eliminated in order to ensure a long shelf life.
In European publication 0 335 381, an organopolysiloxane with modified release behavior and having the general R.sub.a R.sup.1.sub.b SiO.sub.(4-a-b)/2 is described. In this formula, R is a hydrogen group, an alkyl group with 1 to 8 carbon atoms or an aryl group. R.sup.1 is a monovalent phenolic group with 6 to 26 carbon atoms; a is 0, 1 or 2; b is 1, 2 or 3 and a+b is 1, 2 or 3. The organopolysiloxane furthermore contains units of the formula R.sub.a R.sup.2.sub.c SiO.sub.(4-a-b)/2' in which R and a are defined as above. R.sub.2 is a hydrogen group, a monovalent organofunctional acrylic group or a monovalent organofunctional epoxy group; c is 1, 2 or 3 and a+b is 1, 2 or 3. The release power of such a siloxane is modified by the amount of monovalent phenolic groups contained. The higher the content of phenolic groups, the lower are the release properties of the siloxane. However, as a result of the high acidity of the phenolic hydroxyl group, considerable problems arise with the synthesis and storage of these modified siloxanes. The epoxy groups can be activated to polymerize by the phenolic OH group and this leads to unwanted gelling and premature curing of the modified siloxanes.